The present invention relates, in general, to a lighting apparatus using light-emitting diodes (LEDs) and, more particularly, to an LED lighting apparatus having improved flicker performance which is driven directly by a rectified voltage without using a typical switching mode power supply (SMPS).
Light-emitting diodes (LEDs) are electrophotonic conversion semiconductor devices that emit light when current flows therethrough, and are widely used for indicators, backlights, etc. The electrophotonic conversion efficiency of LEDs is higher than that of incandescent lamps and that of fluorescent lamps in response to the development of technologies, and their range is expanding to general lighting systems.
Among methods of driving an LED, a plurality of methods of driving an LED lamp using a rectified voltage without a typical switching mode power supply (SMPS) (hereinafter referred to as “LED direct drive methods”), including Patent No. 10-1110380 of the inventor, was introduced.
Reference will now be made to conventional methods of driving an LED using a DC voltage with reference to FIG. 1 to FIG. 4.
<Related Art 1>
As shown in FIG. 1, a conventional LED lighting apparatus includes an alternating current (AC) power supply 910 which supplies an AC voltage, a rectifier circuit 940 which converts the AC voltage into a DC rectified voltage Vrect, an LED block 970 which is a load to be driven in response to an output from the rectifier circuit 940, and a current limiting device 930 which limits a current flowing through the LED block 970.
However, in the conventional LED lighting apparatus, no current flows at or lower than the threshold voltage of the LED block 970. The LED block 970 which is an electrophototic conversion device does not emit light in this voltage range but emits maximum light at the maximum instantaneous rectified voltage. Consequently, the light output is not uniform and fluctuations with the time, which is problematic.
The problem will be described in detail with reference to FIG. 2 and FIG. 3.
In FIG. 2, a current-voltage characteristic curve 950 is a characteristic curve of an AX2200, an AC drive LED device available from Seoul Semiconductor Co. Ltd. Since the AX2200 is a device that operates with AC power, an LED lighting apparatus using this device does not need a rectifier circuit 940. However, since the same profile of the current-voltage characteristic curve is identical to that of a typical diode characteristic curve (the current increases exponentially while the voltage increases linearly), the characteristic curve of the AX2200 is used herein for the purpose of numerical explanation. (In FIG. 2, the horizontal axis is effective voltage, and the vertical axis is effective current. Herein, in the description of the concept of the present invention, the axes are set as instantaneous voltage and instantaneous current, respectively, for the sake of convenience of explanation.)
In FIG. 2, it is apparent that the threshold voltage at the current-voltage characteristic curve 950 is 62.5 V. Each of a first linear model 951 and a second linear model 952 is produced by modeling the characteristic curve 950 into a straight line in a simple manner. The first linear model 951 is usable when modeling the instantaneous rectified voltage Vrect varying in the range from 0 to 112.5 V. It is apparent that the current is 0 mA at 62.5 V and 31 mA at 112.5 V. In addition, the second linear model 952 is usable when modeling the instantaneous rectified voltage Vrect varying in the range from 0 to 87.5 V. It is apparent that the current is 0 mA at 62.5 V and 11 mA at 87.5 V.
FIG. 3 shows an example in which the first linear model 951 and the second linear model 952 are applied to a power frequency of 50 Hz.
First, in the first linear model 951 in which the maximum rectified voltage 112.5 V is applied, the rectified voltage Vrect is presented with a waveform 951V, and the rectified current is presented with a waveform 951A. In the second linear model 952 in which the maximum rectified voltage 87.5 V is applied, the rectified voltage Vrect is presented with a waveform 952V, and the rectified current is presented with a waveform 952A.
Since only the size of an input rectified voltage is changed while the LED block 970 is unchanged, the threshold voltage of the LED block is equally 62.5 V but the lighting start time of the LED block 970 is earlier as the effective value of the rectified voltage Vrect increases. For instance, at a power frequency of 50 Hz and maximum rectified voltages 87.5 V and 112.5 V, the points of time when passing through the threshold voltage 62.5 V of the LED block 970 are calculated 2.53 ms and 1.87 ms, respectively. These points of time are converted into rectified voltage phases 45.5° (=2.53/5×90) and 33.7° (=1.87/5×90).
That is, when the maximum rectified voltage is supplied at 87.5 V, the rectified voltage is equal to or lower than the threshold voltage of the LED block 970 before the rectified voltage phase 45.5°. Consequently, no current flows and thus no light is emitted. In addition, when the maximum rectified voltage is supplied at 112.5 V, the rectified voltage is equal to or lower than the threshold voltage of the LED block 970 before the rectified voltage phase 33.7°. Consequently, no current flows and thus no light is emitted.
In addition, at the rectified voltage phase 90°, the maximum currents flow, as presented with the current waveform 952A and the current waveform 951A.
Summarizing FIG. 3, the higher the effective value of the rectified voltage Vrect is, the earlier the lighting start time of the LED lighting module becomes. This consequently increases the lighting time. However, since no light is emitted at a voltage equal to or lower than the threshold voltage of the LED block 970, there is a section where the minimum instantaneous light output is zero.
<Related Art 2>
FIG. 4 is a view quoted from Korean Patent No. 10-1110380 of the inventor. Describing the characteristics of FIG. 4 in relation to the present invention, 1) the conventional LED block 970, namely the load, is divided into a number of sub-light emitting blocks (i.e. a first light emitting block 10, a second light emitting block 11 and a third light emitting block 12). 2) A parallel switch block (including a first switch S11 and a second switch S12) and a controller 4 are provided, which adjust the number of the lighted sub-light emitting blocks by changing the path along which a load current flows depending on the instantaneous voltage. 3) In addition, the load current was limited using a current limiting device CS2.
When the instantaneous voltage is low, a small number of light emitting blocks which are arranged in series is driven. Then, the threshold voltage of the LED blocks, which serves as a load, is lowered below that of the Conventional Method 1. Since the current flows at a relatively earlier voltage phase, the time during which no light is emitted from the LED block is reduced.
Referring to the case in which one sub-light emitting block is lighted, no light is emitted at a voltage equal to or lower than the threshold voltage of the sub-light emitting block. Therefore, the section where the minimum instantaneous light output is 0 is not removed, which is problematic.